One, if you’re using ChefDK, but also have a different ruby install, the gem install command could be installing the gem to a different ruby/gems installation. If that’s the problem, you can use chef gem install to install gems to the right ruby, or use chef shell-init to have chefdk “take over” your shell session, in which case you will be using the correct gem command.

Another reason could be that you used knife rehash to cache the knife commands (which improves knife’s performance). In that case, run knife rehash again.